1. Field of the Invention
The present invention relates generally to the field of direct oxidation fuel cells and, more specifically, to delivery of oxygen to the cathode of the fuel cell.
2. Background Information
Fuel cells are devices in which an electrochemical reaction is used to generate electricity. A variety of materials may be suited for use as a fuel depending upon the materials chosen for the components of the cell. Organic materials, such as methanol or natural gas, are attractive choices for fuel due to the their high specific energy.
Fuel systems may be divided into xe2x80x9creformer-basedxe2x80x9d (i.e., those in which the fuel is processed in some fashion before it is introduced into the cell) or xe2x80x9cdirect oxidationxe2x80x9d in which the fuel is fed directly into the cell without internal processing. Most currently available fuel cells are reformer-based fuel cell systems. However, fuel-processing requirements for such systems limits their use to relatively large applications.
Direct oxidation fuel cell systems may be better suited for a number of applications such as smaller mobile devices (i.e., mobile phones, handheld and laptop computers), as well as in larger applications. Typically, in direct oxidation fuel cells, a carbonaceous liquid fuel in an aqueous solution (typically aqueous methanol) is applied to the anode face of a membrane electrode assembly (MEA) containing a protonically-conductive but, electronically non-conductive membrane (PCM) using a catalyst on the surface of the PCM (or otherwise present in the anode) to enable direct oxidation of the a fuel on the anode. The hydrogen protons are separated from the electrons and the protons pass through the PCM, which is impermeable to the electrons. The electrons thus seek a different path to reunite with the protons and travel through a load, providing electrical power.
One example of a direct oxidation system is the direct methanol fuel cell system or DMFC. In a DMFC, methanol in an aqueous solution is used as fuel, and oxygen, preferably from ambient air, is used as the oxidizing agent. There are two fundamental reactions that occur in a DMFC which allow a DMFC power system to provide electricity to power consuming devices: the anodic disassociation of the methanol and water fuel mixture into CO2, protons, and electrons, and the cathodic combination of protons, electrons and oxygen into water. The overall reaction may be limited by the failure of either of these reactions to completion (i.e. failure to oxidize the fuel mixture will limit the cathodic generation of water, and vice versa).
As noted, the DMFC produces carbon dioxide as a result of the reaction at the anode. This carbon dioxide is usually treated as a waste product, and is separated from the remaining methanol fuel mixture before such fuel is re-circulated.
In an alternative usage, the carbon dioxide gas can be used to passively pump liquid methanol into the fuel cell. This is disclosed in U.S. patent application Ser. No. 09/717,754, filed on Dec. 8, 2000, for a PASSIVELY PUMPED LIQUID FEED FUEL CELL SYSTEM, which is commonly owned by the assignee of the present invention, and which is incorporated by reference herein in its entirety.
Fuel cells have been the subject of intensified recent development because of their high energy density in generating electric power from carbonaceous fuels. This has many benefits in terms of both operating costs and environmental concerns. Adaptation of such cells to mobile uses, however, is not straightforward because of technical difficulties associated with reforming the hydrocarbon fuel in a simple and cost effective manner, and within acceptable form factors and volume limits. Further, a safe and efficient storage means for hydrogen gas presents a challenge because hydrogen gas must be stored at high pressure and at cryogenic temperatures or in heavy absorption matrices in order to achieve useful energy densities. It has been found, however, that a compact means for storing hydrogen is in a hydrogen rich compound with relatively weak chemical bonds, such as methanol (and to a lesser extent, ethanol, propane, butane and other carbonaceous liquids). Thus, efforts to develop the DMFC commercially have increased over the past several years.
However, to ensure that the DMFC will continue generating electricity, sufficient oxygen must be supplied to the cathode, and under certain operating conditions, it may be necessary to provide a means to facilitate the removal of excess water from the fuel cell.
There remains a need, therefore, for fuel cell system that optimizes oxygen being provided to the cathode and avoids excess water accumulation on the cathode of the fuel cell.
It is thus an object of the present invention to provide a method and apparatus for oxygen management in a fuel cell system. It is a further object of the invention to provide oxygen management while utilizing by-products of the electrochemical reactions in the fuel cell to maintain a sufficient flow of oxygen across the cathode in a manner that minimizes the use of electricity from the fuel cell.
It is a further object of the invention to provide an air management system that continues to operate efficiently in various physical orientations, and that is adjustable for variations in operating conditions, such as for example, changes in temperature or relative humidity of the ambient environment.
The present invention provides a method and apparatus for oxygen management in a direct oxidation fuel cell system. The oxygen management apparatus forces oxygen (typically from ambient air) through the cathode chamber to facilitate the flow of oxygen across the cathode face of the membrane electrode assembly (MEA) of the fuel cell. It does so by utilizing anodically-generated carbon dioxide to cause the flow of ambient air or other oxidizing agent across the cathode face of the MEA.
In a first embodiment of the invention, a turbine assembly is placed in fluid communication with the anode side of the fuel cell. The turbine assembly is driven when the carbon dioxide produced at the anode side flows over the blades of a first turbine (which is referred to herein as xe2x80x9cthe vent turbinexe2x80x9d). The vent turbine is attached to a drive shaft that is used to cause a second turbine (or fan) to draw oxygen (generally from ambient air) into the cathode side of the fuel cell. This drawn-in air flows over the cathode face of the membrane electrolyte assembly of the fuel cell. The second turbine is referred to herein as xe2x80x9cthe inlet turbine.xe2x80x9d
In general, there is a direct relationship between the amount of anodically-generated carbon dioxide and the oxygen required to complete the cathodic reaction. Further, the amount of air forced over the cathode (which is driven by the anodically-generated CO2) depends upon the amount of CO2 that is generated. It follows then that as more CO2 is generated, more oxygen will be forced over the cathode. Accordingly, the oxygen management can occur passively in this embodiment of the invention.
In order to maintain greater control over the air flow while still having a passive system, in another embodiment of the invention the CO2 is collected in a chamber before passing to the vent turbine. While CO2 is generated, it accumulates in the chamber until a sufficient volume is generated to provide pressure to drive the vent turbine at a desired rate. This rate can be regulated in a number of ways including measuring the gas pressure and actuating a valve to release the CO2 when a sufficient amount of CO2 has been generated or is at a desired value. Alternatively, a pressure release valve may be used to release the stored CO2 to drive the vent turbine.
The system is also adjustable for conditions such as damp environments and/or low ambient temperatures. In such cases, it is possible that the CO2 generated will not be sufficient to drive the vent turbine at a rate that will supply sufficient oxygen to maintain the desired cathodic reactions. In order to address these situations, the fuel system of the present invention includes a motor to actively drive either the vent turbine or the inlet turbine, or both. This may be needed in such damp environments where increased air-flow is needed to remove excess water from the cathode chamber. Alternatively, in warm or dry conditions, it may be necessary to limit the flow of air across the cathode to prevent the membrane electrolyte from drying out, in which case, the membrane electrolyte may not operate properly.
In accordance with another aspect of the invention, a diaphragm is incorporated into the membrane electrode assembly to provide airflow management. As noted herein, the anode chamber and the cathode chamber are separated by a membrane-diaphragm assembly. A first portion of the membrane-diaphragm assembly consists of a membrane electrode assembly. In accordance with this aspect of the invention, a second portion of the membrane consists of a gas impermeable flexible diaphragm. Additionally, the cathode chamber has an air inlet valve and an air outlet valve, each of which may be controlled using a digital controller. In an air inlet mode, the inlet valve is open and the outlet valve is closed, allowing fresh air to enter but not to exit the cathode chamber. Carbon dioxide generated at the anode is vented to the environment (or is used to drive other processes in the fuel cell system). In the air displacement mode, the inlet valve is closed while the outlet valve is open and the anodic generation of CO2 increases the pressure in the anode chamber causing the diaphragm to expand thus driving air out of the cathode chamber and causing air to flow over the cathodic face of the membrane electrolyte assembly. The outlet valve is then closed and the anode valve opened and the CO2 is vented. The system is thus returned to the air inlet mode. Upon return to the air inlet mode, fresh air returns to the cathode chamber to provide oxygen to facilitate the reaction. The sequence is repeated as needed. Alternatively, the MEA may be constructed of flexible gas impermeable materials that allow the MEA itself to act as a diaphragm, thus facilitating the flow of air within the cathode chamber of the fuel cell system.